1. Field of the Invention
The present invention relates to zoom lenses capable of compensating for the change of the angle of field of view due to focusing and, more particularly, to zoom lenses of which at least a part of the front lens members constituting a first lens unit, when counted from the object side, is used for focusing, wherein the change of the image angle resulting from the focusing is corrected by utilizing a part of the zoom section in part (variator) while still maintaining the high zoom ratio.
2. Description of the Related Art
The recent trend in the field of color television cameras for broadcasting is to lay emphasis on handling and manageability. To assist in fulfilling this demand, the image pickup device has been reduced in size. So, the use of small-sized CCD (solid-state image sensor) to, for example, 2/3 or 1/2 inch comes to a main stream. With the help of this, it becomes easy to achieve much desired reduction of the size and weight of the entirety of the camera system.
Along with this has even come a corresponding demand for zoom lenses to be used in the color television camera for broadcasting. So, attempts have been made to reduce the bulk and size and the weight of the zoom lens and, at the same time, to provide the zoom lens with higher specific capabilities.
Among others, to shorten the distance representing how close the television camera can approach an object being shot ahead thereof, i.e., the so-called M.O.D. (Minimum Object Distance) is coming to be one of the important elements on the specification of the zoom lens to be used in the color television camera for broadcasting and on its imaging effect.
However, as this M.O.D. is shortened, the problem that the image angle (the angle of field of view) varies with focusing becomes serious. This is particularly prominent in the type of zoom lens which employs the focusing method using the first lens unit, when counted from the object side, (or the front lens members).
Such a variation of the image angle is in principle based on the following reason. Although the front lens members axially move in part or as a whole, an object point for the subsequent zooming lens unit, or the variator, is maintained in a constant position.
With this, when the object distance changes, or the focusing lens moves forward, the lateral magnification of the front lens members is caused to change. As a result, the lateral magnification of the entire system of the zoom lens changes, which induces a change of the image angle.
For the zoom lens employing this front focus method, it optically happens in the wide-angle positions that the forward movement of the focusing lens, because of little changing of the degree of focus, makes only the change of the image angle to be appreciable. For this reason, a problem arises in that when a change of the image angle due to focusing occurs, it is perceived as if zooming were done.
So, in, for example, U.S. Pat. No. 4,083,057, there has been proposed a device for preventing such change of the image angle from taking place by making movable the zoom section. For example, the lens unit for varying the focal length (variator) is chosen to compensate for the change of the lateral magnification of the entire system of the zoom lens resulted from focusing. For this purpose, the variator is arranged to vary its lateral magnification independently. Thus, the change of the image angle due to focusing is corrected.
In application to a zoom lens comprising, from front to rear, a focusing lens unit (front lens members), a zooming lens unit for varying the focal length (variator), another zooming lens unit (compensator) and an image forming lens unit (relay lens), or the so-called 4-unit zoom lens, it is, however, manifest in the general case that when, for example, in the wide angle positions, the front lens members form an image point very far from an axial position of the variator and this is disadvantageous from the standpoint of correcting the change of the image angle due to focusing. For this reason, if the variator is used in varying the lateral magnification of the entire system, the required amount of correcting movement of the variator becomes very large. This gets even more prominent as the maximum image angle increases, or the zoom lens is a wide-angle one.
Usually, in the 4-unit zoom lens, the wide-angle end becomes a zooming position that puts the front lens unit and the variator at a closest distance from each other. For this reason, to ensure a sufficient space in which the variator moves to compensate for the change of the image angle in that zooming position or the wide-angle end, there is a need to make sure that the interval between the principal points of the front lens unit and the variator be made wider than is usually necessary. An alternative measure is to sacrifice the reduction of the shortest focal length of the entire system.
However, the increase of the interval between the principal points of the front lens unit and the variator calls for an increase of the diameter of the front lens members, giving rise to a problem that the camera apparatus as a whole gets larger in size and heavier in weight.
Also, if the reduction of the shortest focal length is sacrificed, leaving it long, another problem arises in that it becomes impossible for the zoom lens in itself to achieve widening of the maximum image angle and increasing of the zooming range.
Meanwhile, zoom lenses for the color television cameras for broadcasting have, in most cases, zoom ratios of 10 or above. Again, as compared with the zoom lenses for video cameras for home use, they have a wider maximum image angle. In addition, for the wide-angle end, they produce an optical effect that, while the F-number is very luminous, the depth of field becomes deeper.
As a result, the change of the image angle with focusing falls within the depth of focus, leaving the image sensed by the pickup means as not in a defocused state. For the picture composition the photographer intends, on the other hand, a discrepancy is formed to the taken image angle, thus giving rise to a problem. For this reason, the fact that the image angle varies with focusing has been said to be unfavorable to the photographer.